Agglomerate of iron ore

ABSTRACT

A binder for use in agglomerating finely divided iron ore materials, e.g., concentrates, is prepared from an iron-bearing material known on the Mesabi Range of Minnesota as &#39;&#39;&#39;&#39;paint rock.&#39;&#39;&#39;&#39; This is a red or orange, fairly soft and extremely sticky mineral consisting essentially of hematite, kaolinite and quartz, and having a high-ignition loss and high-moisture content. This naturally occurring material is dried-to a moisture content of about 6 percent or lower-at a temperature too low to drive off combined water; and ground to about 200 mesh. The ground dry paint rock is added to and thoroughly mixed with moist iron ore fines (e.g., to moist filtercake of iron ore concentrate), and the mixture is thereafter fed to any conventional balling device where it is rolled into small &#39;&#39;&#39;&#39;pellets&#39;&#39;&#39;&#39; preliminary to being indurated. The present invention relates to the art of beneficiating ore materials, particularly iron ore materials, and is concerned with improvements in pelletizing ore material fines, particularly iron ore concentrates.

United States Patent {72] Inventor Fred D. DeVauey Duluth, Minn. [2!]Appl. No. 787,209 [22] Filed Dec. 26, 1968 [45] Patented Dec, 21, 1971[73] Assignee The Shenango Furnace Company Contlnuation-in-part ofapplication Ser. No. 666,541, Aug. 24, 1968, now abandoned. Thisapplication Dec. 26, 1968, Ser. No. 787,209

[54] AGGLOMERATE OF IRON ORE 1 Claim, No Drawings [52] U.S. Cl 75/3 [51]Int. Cl C211: 1/24 [50] Field of Search 75/1, 3-5

[56] References Cited UNITED STATES PATENTS 2,336,618 12/1943 Jones...75/3 2,789,894 4/1957 DeVaney 75/3 3,053,648 9/1962 Stephens,.lr. et al.75/3 X 3,097,945 7/1963 Paris et a1. 75/3 3,254,985 6/1966 75/33,323,901 6/1967 Dahl et al. 75/3 Primary Examiner- Allen 8. CurtisAttorney-Pierce, Scheffler & Parker ABSTRACT: A binder for use inag'glomerating finely divided iron ore materials, e.g., concentrates, isprepared from an iron-bearing material known on the Mesabi Range ofMinnesota as paint rock." This is a red or orange, fairly soft andextremely sticky mineral consisting essentially of hematite, kaoliniteand quartz, and having a high-ignition loss and highmoisture content.This naturally occurring material is driedto a moisture content of about6 percent or lower-at a temperature too low to drive off combined water;and ground to about 200 mesh. The ground dry paint rock is added to andthoroughly mixed with moist iron ore fines (e.g., to moist filtercake ofiron ore concentrate), and the mixture is thereafter fed to anyconventional balling device where it is rolled into small pellets"preliminary to being indurated.

The present invention relates to the art of beneficiating ore materials,particularly iron ore materials, and is concerned with improvements inpelletizing ore material tines, particularly iron ore concentrates.

AGGLOMERATE OF IRON ORE This application is a continuation-in-part ofapplication Ser. No. 666,541 filed Aug. 24, 1968, now abandoned.

The present invention relates to the art of beneficiating ore materials,particularly iron ore materials, and is concerned with improvements inpelletizing ore material fines, particularly iron ore concentrates.

Pelletizing of iron ore is a relatively recent development but since thefirst commercial plants were built in 1948, the industry has grown byleaps and bounds. In the year 1967 the production of pellets had grownin North America alone to approximately 56,000,000 long tons. Theprocess has been so widely accepted because it made possible theagglomeration of fine concentrates which could not be efficientlyagglomerated by any of the known methods such as sintering ornodulizing. The acceptance of the process was also accelerated by thefact that because pellets were high in iron content, uniform in gradeand size and readily reducible, their use made it possible todrastically increase the capacity of an iron blast furnace and also todecrease the amount of coke and flux required.

The development of successful pelletizing processes was not an easy oneand required years of work by many research laboratories and theexpenditure of many millions of dollars. Without exception the best wayto make pellets has been to take the filtered fine iron concentrates orground wetted iron ore and roll it into balls prior to indurating. Theproper amount of moisture to make a good strong ball that would standthe considerable amount of conveying and mechanical handling incidentalto the indurating process varies with the mineralogical composition andthe size consist of the mixture. The amount of surface area of theground material is of particular importance. The proper amount ofmoisture to render the material ballable, sufficiently plastic so theballs will deform slightly rather than break in handling, and alsosufficiently strong to withstand a moderate load without breaking,varies with this surface area. For example, with a heavy, relativelycoarse mineral like magnetite ground to 65 mesh, the optimum moisturecontent may be as low as 8 percent. With a fine, low density materialsuch as a cement mix, the optimum moisture may be as high as percent.For most iron ore concentrates the optimum moisture range is between 8.5and 10.5 percent. The actual balling operation may be carried on in avariety of production machines such as balling drums, balling cones orballing discs. 7 V 7 The formed and closely sized damp balls may then beindurated by a variety of pelletizing methods. Furnaces now in generaluse consist of three general types. These are:

1. The vertical shaft furnace;

2. The continuous grate machine;

3. The third type of machine is the so-called grate-kiln machine.

One of the problems that troubled all early investigators ofpelletizing, irrespective of the type of furnace used, was that themoist green balls, while fairly strong in the moist stage, became quiteweak structurally during the early stages of heating and broke due tothe mechanical stresses imposed by static loading or by handling.However, a much more serious problem was the explosion or exfoliation ofthe balls during the initial induration stage when the moisture wasbeing driven from the balls. The rate of heating during the indurationstages may be extremely rapidapproaching 200? F. per minuteand becausethe balls are normally weak as the moisture is driven Off, nxbal q a lyexa qslstt .darl slhs nit a heating stage and were reduced to dust. Thepelletizing process would never have attained success unless a solutionto this problem was solved.

The solution to this problem was the finding of additives which wouldmake the green balls more plastic and more easily handled withoutbreakage prior to firing and which gave the balls an added bond duringinduration so that they would not exfoliate or explode prior to the timewhen they acquired a high strength at temperatures in the 2300 to 2600F. range, through either grain growth or slag bonding.

Prior to 1951 two types of additives were found that solveddifficulties. The first additive successfully used was starch at thepilot plant of Erie Mining Co. at Aurora, Minnesota in 1950 and thisproved to be a major advance in the art. However, because the price ofstarch was about 7 per pound and 3 pounds were required per ton of theconcentrates, a search for a cheaper additive was instituted. Later in1950 at the same plant, bentonite was found to be a cheaper and moreeffective additive. The use of both starch and bentonite were developedand patented by the applicant. The use of starch is described in US.Pat. No. 2,595,132, May 13, 1952. The use of bentonite is described inUS Pat. No. 2,743,172, Apr. 24, 1956. The effectiveness of bentonite andthe readiness with which the pelletizing industry has accepted its usemay be judged from the fact that in 1966 some 500,000 tons of groundbentonite was used in North American plants pelletizing iron ores. I

While the use of bentonite has proved to be a success in the pelletizingof iron ore, it nevertheless has some drawbacks. Bentonite usuallycontains about 55 percent silica, and when added to iron concentrate inthe usual amounts of from 15 to 10 lbs. per ton of the concentrates, itraises the silica content of the resultant pellets by about 0.35 percentsilica. This is undesirable, because silica is the chief impurity whichthe concentration process is devised to eliminate. lts inclusion in thepellet definitely lowers the value of the pellet. Bentonite containspractically no iron; hence it acts as a diluent that detracts from thevalue of the finished product. Another disadvantage is that no bentoniteof commercial value occurs near the Lake Superior iron districts withthe result that bentonite has to be shipped long distances from itsnearest sources, i.e. either from the Black Hills area of South Dakotaor adjoining areas in Wyoming. The rail haul to the Lake Superior pelletplants is therefore from 700 to 1000 miles, resulting in high freightrates in the order of about $10.00 per short ton. This freight rate isas high or higher than the cost of the prepared bentonite at itsshipping point.

The search therefore has continued for an additive that could beobtained closer to the large pellet plants in the Lake Superior area andthus reduce freight costs and for one that would not materially increasethe percentage of silica in the pellets. A substitute meeting theserequirements and one which will also result in substantial savings tothe operator of a pelletizing plant has now been found. The material isa particular type of iron-bearing material, or ore," consisting of ironoxide and kaolinite, with or without quartz, the iron oxide content ofwhich is very fine grained and which is high in alumina content andwhich mayand usually doehave a high content of moisture in its naturalstate. Included in this definition is a material occurring on the MesabiRange of Minnesota, United States of America and which is known as paintrock." The material is found also in other areas, such as the LabradorTrough, and is known by other names as well.

PAINT ROCK Paint rock is the name given to an iron-rich material in theBiwabik lron Formation on the Mesabi Range of Minnesota that has beenfound through the oxidation and enrichment of the original slatymembers. A number of paint rock zones may occur in a cross section ofthe iron formation but the main zone occurs as an oxidation remnant ofthe intermediate slate which occurs as the lower geological member inthe Lower Slaty formation. The Lower Cherty formation lies directly 6Q!li Paint rock, as the name indicates, is a red or orange, fairly soft,extremely sticky material. Mineralogically, it consists mainly ofhematite, kaolinite and quartz. A typical analysis of paint rock is thatof a material from the Whiteside Mine of the Snyder Mining Company nearBuhl, Minnesota. An analysis of this material on a dry basis is asfollows:

lron

Phosphorus. Silica. Manganese. Alumina,

Loss hyignition Moisture Mineralogically, the above sample consistsapproximately of 75 percent hematite, 20 percent kaolinite and percentquartz. Other operable paint rocks may have varying amounts of thesethree constituents, the hematite ranging from 60 to as much as 80percent and the kaolinite ranging from to 35 percent. For the presentpurpose, the paint rock with the highest kaolinite (and the lowestquartz) content is the most desirable. lts mineral particles areexceedingly fine since they are derived from the alteration of slates.The extreme sticki ness or adhesiveness of the paint rock is undoubtedlydue to its large content of kaolinite (the common name for clay), and tothe extremely fine grain sizes of the hematite and the kaolinite. Theaverage Blaine number of paint rock, ground to 94 percent 200 meshTayler, is 9,900 sq. cm. per gram, (or, about 10,900 when ground to 98percent minus 200 mesh)- more broadly, within the range 8,000-1 l,00O-,which compares with an average Blaine number of 3,180 for bentoniteground to 85.2 percent minus 200 mesh Tyler and an average Blaine numberof 1600-2100 for a taconite concentrate 85-95 percent by weight of whichis minus 325 mesh Tyler. This rock, because of its origin, isparticularly fine-grained and thus particularly effective as anadditive.

Reference: Gruner, J. W., Mineralogy and Geology of the Mesabi Range;Minnesota Geological Survey, 1946.

The paint rock on grinding, and later dispersion in water, breaks downto an exceedingly fine sticky claylike material. The adhesivelikeproperties of this material have been known for many years and there hasbeen some consideration for its use as an additive in pelletizing. Someattempts along this line have been made in the past but without successand have been abandoned. it has only been recently that more detailedstudy has shown that this material can be an effective binder, but onlywhen it has been prepared and used under carefully controlledconditions. The preparation of the paint rock and its. method ofapplication constitute this invention.

Earlier attempts to use paint rock failed because of the inability tosatisfactorily wet grind and filter the paint rock. Other attempts touse coarser ground material likewise failed because of the relativelysmall amount of new surface produced to act as a binder. Attempts at drygrinding also failed probably because of improper drying and grinding.

l have found that the steps of drying and grinding are all of majorimportance and if either step is not carried out in the manner whichconstitutes this invention the additive will be ineffective.

I have found that the first step in the preparation of the paint rock,after mining and coarse crushing to about 4 inches, is the dryingoperation which must be done carefully. I! may be noted that in thetypical analysis of paint rock given hereinbeiore, the material is highin moisture (l9.7 percent) and also has a high "Loss on ignition" 10percent). This Loss on ignition is almost entirely made up of thecherniculiy combined water. either as water olcrystallizntion or us thehydroxyl molecule present in the hydrous minerals in the paint rock, theprincipal one of which is kaolinite. I discovered that not only is itdesirable to dry this paint rock so that the amount of moisture isreduced to below 6 percent but also that in so doing it is imperativethat none of the paint rock be subjected to a temperature at which thecombined water will be driven off. If any part of this ore material isheated above the temperature at which combined water is driven off, eg,as high as about l500 F., the nature of the mineral is changed. if suchdehydrated mineral is brought again in contact with water, it will notretake water but will remain inert plastically and will be useless as abinder.

Mention has been made that the moisture should be reduced to below 6percent. The absolute figure will vary with the type of paint rock used,it is, of course, necessary that the ground material be not sticky sothat it will not hang up in bins or conveyor pipes. Overdrying beyondthis simply means additional expense. However, drying of some types ofpaint roclr. down to l or 2 percent moisture may be necessary to secureefiicient fine grinding and the avoidance of caking in grinding mills,pipes and storage bins. As a matter of practicality, I prefer drying intwo stages-the first stage to be carried out in a rotary dryer withcareful temperature controls. The second step combines the fine grindingdown to 200 mesh stage with the final drying operation in a hotair-swept ball mill or equivalent grinding device.

The customary method of adding additives to a fine iron concentrate isto meter the dry additive and the filtered concentrate and send them toa mixing device such as a mixermuller. Here the two are mixed and insome plants any additional water required for balling is added. Themixture then goes to a balling device such as a balling drum or disc.The retention time in either device is very short. Even with a ballingdrum with a circulating load of 200 percent the total elapsed time fromthe mixing of the additive with the concentrate to the charging of theballs into the furnace is only about 4 minutes. This is an extremelyshort time for any dry additive to absorb its full complement ofmoisture, either from the damp concentrate or from the small amount ofadded water. i have found that with both bentonite and paint rock asubstantial gain in the utilization of the resulting product can besecured if the dry additive is mixed with the moist concentrate in thedescribed manner and then, prior to balling, held (stored") for a timesufiicient to permit the additive to absorb its full complement ofwater. During this storage period the additive takes up its fullcomplement of water and achieves its max-- imum plasticity and bindingpower. Another advantage occurs in that a much more uniform feed as tosize consist, chemical and moisture content is produced, which resultsin better and more uniform pellets. While this storage is helpful in thecase of paint rock-just as in the case of bentonite-it is not essentialto the successful use of the material as a binder.

One of the main advantages of using paint rock is that the paint rockhas a substantial iron content. When added to taconite (or other oxidiciron ore) concentrates to be pelletized, a substantial gain in the totaltonnage of pellets results with little change in the grade of theproduct. Because the paint rock contains only from 6 to ID percent SiOthere is no marked increase in the silica content of the pellet asobtains in the case of bentonite which contains from 50 to 60 percentsilica.

A better understanding of the value of paint rock as an additive can beobtained through an actual example ofits use.

EXAMPLE NO. l

in this example the starting concentrate was a high-grade magnetiteconcentrate from an Ontario property just north of Lake Superior. Tothis dried concentrate was added 4 percent dry weight of paint rock oredried and ground in the manner just described. These two materialstogether with an amount of water were mixed and mulled together andstored over night (eight or more hours) prior to balling. Similarly, alike amount of the same concentrate was mixed with 0.5 percent of a goodquality swellingtype Black Hills bentonite, stored and balled in thesame manner. After balling to two types of pellets were fired togetherin the same pelletizing pot separated only by a divider so that thefiring conditions were identical. A summary of this test is shown on theattached table l. In each case, the particle size of the additive (paintrock; bentonite) was about percent minus 200 mesh Tyler.

TABLE N0. I.PAINI ROCK VS. BENTONITE Additive in Pelletizing OntarioMagnetitc Results of Firing Tests Sizing analysis of tumbled Compressiysstrength, pellets, percent wt.

Binder Avg Max Min. -l-- )4 28M 28M 4% Paint Rock. 537 1,050 180 82.115.11 2.0 0.5% Bentonite 337 750 160 60. 1 36. 2 3. 7

Concentrates and water mixed and mulled as in other tests butballadimmediately.

Additive Paint Rock [Analysis and weight of materials and products] Wt.Percent percent Material (dry) Fe Phos. Mang. Sllicu Starting Ontarioconct 100.00 60. 61 .000 0.12 2. 50 Paint Rock 4. 00 61. 40 .118 0.1118. 00 Resulting pellets 106. 80 67.03 .013 0. 2. 03

Additive Bentonite [Analysis and weight of materials and products] Wt.Percent percent Material (dry) Fe Phos. Mang. Silica Starting Ontarioconot 100.00 00. 61 .000 0.12 2.50 licntonitc 0. 50 55. 00 103. 65 b7 00000 0.12 2.03

Resulting pellets For the purpose of comparison, the characteristics ofballs made with no additive are also shown. It will be seen that theballs with the paint rock additive definitely have both a higher dry andwet strength than do the balls with no additive. The wet and drystrength of balls with bentonite is higher than those with paint rock;however, the compressive strength of the balls with paint rock ishighenough to meet industrial standards and hence is satisfactory.

As mentioned in a previous paragraph one of the main purposes of usingan additive in pelletizing is to prevent exfoliation or decrepitation.The test data shows that the balls with paint rock are somewhat superiorto those made with bentonite and much superior to those with noadditive. The fired pellets were subject to standard compressive testsand also to standard tumble tests to measure their quality. in thetumble test fired pellets are placed ina drum, fitted with lifters, androtated. At the end of a definite number of revolutions the pellets areremoved and screened. The strength of pellets is measured by the amountsof degradation that takes place. An inspection of this table shows thatthe average fired strength of pellets made with paint rock was over 50percent greater than those made with bentonite. This improved quality ofthe pellets is also evidenced by the fact that after the tumble test82.1 percent of the pellets with paint rock remained coarser thanone-fourth inches whereas only 60.1 percent of the pellets made withbentonite were coarser than this size. This is additional proof that thepellets made with paint rock were definitely stronger than those madewith bentonite. A comparison of the analysis of the starting materialsand for the finished pellets for the described test, together with theweight recovery of the pellets is also shown. It should be noted that inthe pelletizing process the magnetite (Fe,0 in the concentrate isoxidized to hematite (Fe O with a gain in weight. For example, ifpellets were made of pure magnetite (72.3 percent iron) and wereconverted entirely to hematite (70.0 percent iron) there would be a gainin weight of 3.2 percent. A comparison of the two types of pellets showsthat the analyses of the two types of pellets are almost identical butbecause of the greater weight of paint rock added, the weight of thesefired pellets, taking as the starting weight of dry concentrates in eachexample, is 106.80 as compared to 103.65. This is a gain in weight of3.15 percent. This additional tonnage represents considerable addedrevenue. A detailed analysis of the economics involved has shown that ifpaint rock is substituted in place of bentonite as the additive, asaving of about 25 per ton of starting concentrates can be made. Thiscomes about mainly because paint rock is relatively cheap and becauseits iron units have appreciable worth.

EXAMPLE NO. 2

Mesabi taconite concentrate:

Iron 65. 50 Phosphorus 009 Manganese 0. 2:") Silica 7. 30

This test was made on a considerably larger scale than the one detailedin example No. 1. A sample of filtercake containing 9.5 percent moisturewas taken directly from the mill filter and to this was added 3 percentby weight of dried, 200 mesh ground, paint rock obtained from theWhiteside Mine at Buhl, Minnesota. The mixture was thoroughly mixed bypassage over a mixing-mulling device. It was then placed in drums andallowed to stand overnight to give ample opportunity for the mixture tobecome homogeneous relative to moisture. it should be noted that themoisture of production mill concentrate contains somewhat more moisture(9.5 percent) than did the sample treated in example No. 1 (7.8percent). This is typical of the spread in moisture content betweenballs rolled in a production plant as contrasted to those made startingwith dry concentrate in a laboratory. The 9.5 percent moisture wassufficient so that the moisture-accepting capacity of the paint rock wascompletely satisfied. On the day following the mixing of the material,it was bailed in a 4-ft. balling disc. The resulting balls were thenfired in a pot grate furnace having a capacity of 1 cubic ft. A toptemperature of 2550 F. was achieved in this furnace. The fired pelletsafter discharge from the furnace were chemically analyzed and checkedfor quality by standard compression and tumble tests. A number of firingtests were made to detennine the optimum firing cycle. it was found thatthe following cycle gave excellent results with no decrepitation orexfoliation.

a. Updraft Drying pass at 850 F. for 5 minutes, using 300 S.C.F.M./Ft'*of hearth area; b. A Downdraft Drying pass at 7000 F. for 2 minutes, 200

S.C.F.M./Ft'= c. A preheat pass at 1800 F. for 4 minutes;

d. Firing at 2375 F. for approximately 15 minutes.

Tests made on a representative sample of green balls showed them to havean average wet strength of 3.0 lbs. and an average dried strength of 3.8lbs. The average ball could be dropped 8 times from height of 12 inchesbefore breaking. Tests on the fired balls showed an average compressivestrength of 515 pounds. A standard 25-lb. 200-revolution tumbler testshowed 94.5 percent of the tumbled product coarser than 3 mesh. Thesetests showed the product to be of excellent quality and the equal ofpellets made with bentonite.

The additive developed is particularly valuable if used in the LakeSuperior District in the pelletizing of the specular hematite ore ofMichigan and the magnetite concentrates of Michigan, Minnesota andWestern Ontario. These areas are close to the supply of paint rock fromthe Mesabi area. Paint rock is relatively cheap as it is now in littledemand. It is also relatively close to the aboveanentioned districts andfreight costs will be relatively low.

EXAMPLE NO. 3

Test work in Example No. 1 was done on a very fine concentrate, having asizing analysis of 97.4 percent finer than 325 mesh. It was thought thissample was considerably finer than the average Mesabi taconiteconcentrate, and that it was essential to test paint rock on thelatter-smewhat coarser product. A magnetic taconite concentrate from theReserve Mining Company's operation at Silver Bay, Minnesota wasobtained, the same analyzing 89.2 percent finer than 3 25 mesh.

Test material was prepared in essentially the same manner as in exampleNo. 1 above, in that both the bentonite-containing and the paintrock-containing mixtures were stored over night before being balled andthereafter fired. The balled products were evaluated. lt was found thatwith this coarser concentrate the paint rock showed an even greaterimprovement in preventing decrepitation than in the case of the finerconcentrate of example No. 1. With paint rock, 100 percent of thepellets survived at 750 C. heating period, whereas the bentonite samplecould only be heated to 510 C. before decrepitation took place.

EXAMPLE NO. 4

A continuous balling and pelletizing test was made using a grate-kilnmachine at the Allis-Chalmers Laboratory near Milwaukee, Wisconsin,U.S.A. The starting material was a magnetite cone, from the WesternMasabi Range, analyzing (in percentages by weight) 67.36% iron and 4.10%silica and containing 85.7 percent by weight of particles finer than 325mesh Tyler. These tests were made at the rate of approximately l tonconcentrate per hour, and the total test period extended for acontinuous period of about 2.5 days. During this period variousadditions were used. In the first test, 0.7 percent by weight ofbentonite was added to the damp con centrate. The material was balled ona balling disc. The moisture content of the balled material was 9.5percent by weight. The balls as formed were fed to a continuousgratekiln machine wherein the balls were dried and then indurated to atop temperature of about 2425 F.

Thereafter, and under identical conditions of operation, 4.0 percent byweight of paint rock was substituted for the bentonite of the beginningtest. Balling and firing conditions were identical.

The respective products from these two test compared as follows:

TABLE ll Phase A B lndurated Product Bulk Density (lb.lt\.') I325 [33.0indurated Product Strength (1b.) (compression) 700 700 Tumble index 0n3M) 9588 96.7.4 Abrasion Study 0n 28M) 96.24 96.56 Preheat Strength(1b.)

(compression) 2i 7 337 after grate before kiln Bailing Pan Feed Rate(lb/hr.) 1930 2000 Bailing Pan Feed Moisture (kl 8.8 9.6 Green BallMoisture 9.5 [0.2

Green Ball Strengths (compression) (lb) We! 2.7 2.5 Dry I24 8.: l8"Drops as 7.3

These data show that the quality of the fired pellets made with paintrock is as higher as, or higher than, that of the fired pellets madewith bentonite. The compression strength of the partially fired pellets(preheat stage) is 50 percent higher than those made from bentonite. Thewet and dry, green ball strengths are slightly higher when bentonite isused as compared to paint rock, but the values for paint rock are wellabove acceptable limits. The figure for [8 inches drops in dicates thenumber of times a green ball can be dropped, from a height of 18 inches,before it breaks. Both values are well above acceptable limits.

The respective chemical analyses of the two indurated products comparedas shown in table [II following:

TABLE lll The gain in weight, in the case of pellets made with paintrock, was about 3.5 percent, and in addition the iron content was higherand silica content was lower. The time for mixing the addition with thedamp concentrate was essentially the same in both tests, and was ofshort duration. The total elapsed time from the point of addition ofadditive to concentrate to the point of feeding to the grate furnace didnot exceed 5 minutes in either case.

Test work has shown that paint rock as an additive is particularlyuseful in pelletizing the specular hernatite ore of Michigan. These oresare treated by flotation after a desliming operation. The resultingconcentrates are therefore low in colloids which makes balling difficultunless much grinding is done to reduce the size of the flotationconcentrates. Paint rock has an advantage over bentonite as an additivebecause of the greater weight used which increases the percentage ofcolloids in the mix.

The amount of paint rock necessary as an additive will, of course,depend on the physical characteristics of the paint rock used and alsoon the nature of the iron ore concentrates with which it is mixed. Withsome ores an amount of paint rock as low as 1 percent by weight may besufi'rcient as an additive. For most taconrte concentrates an amount offrom 2.5

to 5 percent seems indicated. In areas where paint rock can be producedclose to a taconite plant, such as in areas of the Central Mesabi Range,it may prove economical to use amounts up to 10 percent, not becausethis amount is needed as an additive but because it permits cheap paintrock iron units to be converted to more valuable pellet iron units.

We claim:

1. An agglomerate consisting essentially of iron ore particles and abinder bonding said particles together, said binder consistingessentially of an oxidic iron ore material consisting mainly ofhematite, kaolinite and quartz, substantially all of which binder passesa 200 mesh screen Tyler and exhibits a fineness corresponding to Blaine8000-1 l,000 emf/gram, and in which the surface moisture content is from1 to 6 percent by weight and the loss on ignition is of the order of l0percent by weight.

